12-year-old Regan MacNeil begins to adapt an explicit new personality as strange events befall the local area of Georgetown. Her mother becomes torn between science and superstition in a desperate bid to save her daughter, and ultimately turns to her last hope: Father Damien Karras, a troubled priest who is struggling with his own faith.

Haunted by terrible acts the Nazis forced him to participate in, the disenchanted Father Lankester Merrin focuses his energies on helping with an archaeological dig in the northern part of Kenya. There, the crew uncovers a church that predates Christian missions into the area, but the discovery also causes strange events to start occurring -- including physical changes to a young, disabled boy, whom Merrin grows to suspect has been possessed by a demon.

Seminary student Michael Kovak reluctantly attends exorcism school at the Vatican. While he’s in Rome, Michael meets an unorthodox priest who introduces him to the darker side of his faith, uncovering the devil’s reach even to one of the holiest places on Earth.

On 28 December 1999, the citizens of New York City are getting ready for the turn of the millennium. However, the Devil decides to crash the party by coming to the city, inhabiting a man's body, and searching for his chosen bride—a 20-year-old woman named Christine York. The world will end, and the only hope lies within an atheist named Jericho Cane.

Having lived through traumatizing events during WWII, Father Lankester Merrin takes a sabbatical from the Church to conduct archaeological excavations in British-administered East Africa. Merrin unearths an ancient Byzantine church believed have been built and then immediately buried to keep down evil from the crypt below. The natives are convinced that uncovering the church has unleashed a demon, and begin to violently clash with the British military troops. As the village rapidly disintegrates into chaos and war, Merrin must face-off with the demon which has taken possession of somebody close to him.